This invention relates to an apparatus for producing a DC electrical signal whose level is representative of the magnitude of the most recent peak of a time varying signal.
Peak detectors receive an AC input signal and, in response, provide a DC signal whose level is approximately equal to the amplitude of the AC input. Capacitive peak detectors apply the received AC input to a diode connected in series with a capacitor. When the AC input voltage exceeds the capacitor's voltage, the diode turns on to charge the capacitor up to the voltage of the AC input. Once the AC input reaches its peak and begins falling, the diode shuts off, preventing the capacitor from discharging downward with the AC input.
Since capacitors tend to discharge internally, the capacitor voltage slowly decays below the peak level. However, if the next peak arrives relatively soon, the capacitor voltage will not decay substantially before rising upward with the new peak. Accordingly, if the frequency of the input signal is high relative to the rate of the capacitor's discharge, the capacitor will provide a DC voltage whose level is approximately equal to the magnitude of the most recent peak of the AC input.
If the amplitude of a low peak of the AC input is less than the capacitor voltage, the diode will remain off and the capacitor voltage will not change in response to the arrival of the peak. Since the capacitive peak detector has no mechanism for changing the capacitor voltage downward to match the relatively low peak, the capacitor voltage level will not be representative of the relatively low peaks. Further, if the frequency of the AC input signal is low relative to the rate of the capacitor's discharge, the capacitor voltage may discharge substantially between peaks such that its level between peaks is not representative of the amplitude of the AC input signal.